This invention relates to a method for converting undesirable sulfur compounds in geothermal steam to less volatile and environmentally less objectionable compounds.
The development of geothermal power has been slowed due to the presence of such sulfur compounds as hydrogen sulfide, bisulfide ion and sulfide ion in the geothermal steam produced from drilled wells. When released to the atmosphere, hydrogen sulfide has an objectionable odor detectable in adjacent inhabited areas and is therefore considered to be generally environmentally undesirable or intolerable.
Various processes have been proposed for removing hydrogen sulfide from the steam after it is used and before noncondensed gases are vented to the atmosphere. For example, one such process which has been practiced on power plants using geothermal steam from wells in California, is known to those versed in the art as the "Stretford" process and is described in U.S. Pat. No. 3,097,926. Essentially, in this process, the noncondensable gas stream is treated with aqueous alkaline solution to remove hydrogen sulfide before being vented.
Although the problem of contaminating the atmosphere was relieved by the aforesaid process and other similar processes for removing hydrogen sulfide from the noncondensable gases, the initially condensed steam created a secondary abatement problem in that the aqueous hydrogen sulfide dissolved in the steam condensate was introduced into the circulating water and was ultimately outgassed in the cooling tower. Known treatment processes for discharging hydrogen sulfide converted it into solid residues consisting of insoluble metal sulfides, elemental sulfur, and to a lesser extent, soluble sulfur compounds such as thiosulfate ion and sulfate ion. These solid residues created severe operating problems within the production units of the steam utilization system by plugging process equipments, such as heat exchanger tubes, cooling tower nozzles, and drift eliminators.
It is therefore a general object of the present invention to solve this problem by providing a process and apparatus that converts the aqueous hydrogen sulfide in the steam condensate to compounds of lesser volatility without forming objectionable amounts of insoluble residues. That is to say, the objectionable sulfur compounds, including hydrogen sulfide, bisulfide and sulfide ions, are converted to compounds of such lesser volatility so that gases ultimately vented to the atmosphere meet environmental standards or requirements therefor, and the condensate or circulating water is substantially free of insoluble residues or substances derived from the starting sulfur compounds.
Another object of the invention is to provide a process for converting aqueous hydrogen sulfide from contaminated steam condensate that is relatively easy to carry out with a minimum of specialized apparatus or highly skilled labor and which is therefore relatively inexpensive.